Definitions
According to Arrhenius's original definition, an acid is a substance that dissociates in aqueous solution, releasing the hydrogen ion H+ (a proton):
- HA A− + H+.
The equilibrium constant for this dissociation reaction is known as a dissociation constant. The liberated proton combines with a water molecule to give a hydronium (or oxonium) ion H3O+, and so Arrhenius later proposed that the dissociation should be written as an acid–base reaction:
- HA + H2O A− + H3O+.
Brønsted and Lowry generalised this further to a proton exchange reaction:
- acid + base conjugate base + conjugate acid.
The acid loses a proton, leaving a conjugate base; the proton is transferred to the base, creating a conjugate acid. For aqueous solutions of an acid HA, the base is water; the conjugate base is A− and the conjugate acid is the hydronium ion. The Brønsted–Lowry definition applies to other solvents, such as dimethyl sulfoxide: the solvent S acts as a base, accepting a proton and forming the conjugate acid SH+.
In solution chemistry, it is common to use H+ as an abbreviation for the solvated hydrogen ion, regardless of the solvent. In aqueous solution H+ denotes a solvated hydronium ion rather than a proton.
The designation of an acid or base as "conjugate" depends on the context. The conjugate acid BH+ of a base B dissociates according to
- BH+ + OH− B + H2O
which is the reverse of the equilibrium
- H2O (acid) + B (base) OH− (conjugate base) + BH+ (conjugate acid).
The hydroxide ion OH−, a well known base, is here acting as the conjugate base of the acid water. Acids and bases are thus regarded simply as donors and acceptors of protons respectively.
A broader definition of acid dissociation includes hydrolysis, in which protons are produced by the splitting of water molecules. For example, boric acid (B(OH)3) produces H3O+ as if it were a proton donor, but it has been confirmed by Raman spectroscopy that this is due to the hydrolysis equilibrium:
- B(OH)3 + 2 H2O B(OH)4− + H3O+.
Similarly, metal ion hydrolysis causes ions such as 3+ to behave as weak acids:
- 3+ +H2O 2+ + H3O+.
Read more about this topic: Acid Dissociation Constant
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